1. Field of the Invention
This invention relates to a technique of effectively replenishing a locality where molten metal is being solidified in a die cavity with molten metal by advancing a pressurizing pin into the cavity, thus preventing a shrinkage cavity or like die casting defect that may otherwise be generated in the cast product as a result of shrinkage of metal attendant upon solidification thereof.
2. Description of the Prior Art
A prior art technique pertaining to this technique is disclosed in Japanese Laid-Open Patent Publication No. 57-127569.
In this technique, until solidification of molten metal charged in a die cavity is completed, the die cavity is continuously replenished with molten metal in an extrusion molten metal chamber by an extruding pin, and also the die cavity is continuously replenished with molten metal in a pressurized molten metal chamber by a pressurizing pin.
In this technique, molten metal charged in the die cavity is solidified in a state that a locality where molten metal is being solidified is continuously replenished with molten metal, thus preventing shrinkage cavity or like die casting defect.
In this prior art method, however, the die cavity is continuously replenished with molten metal from the commencement till the completion of the solidification of molten metal in the die cavity. Therefore, the extruding pin and the pressurizing pin should have capacity (i.e., size and stroke) sufficient for the continuous replenishment with molten metal. That is, there is a problem that the extruding pin and the pressurizing pin become large in size. In addition, it is sometimes difficult to secure sufficient stroke or size of the pins depending on the shape of the cast product. In die casting, the possibility of generation of die casting defects is increased in a latter stage of solidifying step. This poses a difficulty of manufacture of a cast product in which the die casting quality of parts which are solidified in the latter stage of the solidifying step is significant.
A technique for coping with the problem noted above is disclosed in Japanese Laid-open Patent Publication No. 4-182053. In this technique, a pressurizing pin is advanced at a low speed into a die cavity with molten metal charged therein, and during this time, the force that is required for the continuous low speed advancement of the pressurizing pin is continuously detected. Upon reaching of a predetermined value by the detected force, the speed of advancement of the pressurizing pin is increased. According to this technique, the status of process of solidification can be grasped from the force necessary for the continuous low speed advancement of the pressurizing pin.
While there is no substantial progress of solidification, the shrinkage of molten metal attendant upon the solidification is not so much, and the molten metal replenishment by the pressurizing pin is not necessary. 0n the other hand, when the replenishment with molten metal by the pressurizing pin is commenced after excessive progress of solidification, there is al ready shrinkage defect generated as a result of solidification. According to the disclosed technique described above, the status of progress of solidification is grasped by causing continuous slow advancement of the pressurizing pin. It is thus possible to replenish with molten metal during the solidification by advancing the pressurizing pin at an adequate timing which is neither too early nor too late.
However, carrying out this prior art technique proves that proper correspondence cannot always be obtained between the force necessary for the continuous slow advancement of the pressurizing pin and the solidification progress status. In other words, even with this system, it is frequently the case that the pressurizing pin advancement timing for the replenishment is too early or too late. In addition, the control of the advancement speed during low speed advancement is very much sophisticated. If the speed is insufficient, the solidification progress status cannot be detected satisfactorily. If the speed is excessive, on the other hand, a major proportion of the advancement stroke of the pressurizing pin has been used in the detection of the optimum timing. That is, it may occur that the pressurizing pin can no longer be advanced when the molten metal replenishment action is really necessary.